This invention relates to a process for selectively alkylating phenols and more particularly to a process for introducing a methyl group into an ortho-position of a phenol. The description of a position is with respect to a hydroxyl group of the phenol.
Ortho-methylphenols, particularly 2, 6-xylenol, are important compounds which are recently attracting wide interest as raw materials of synthetic resins such as polyphenylene-oxide (hereinafter referred to simply as PPO). Furthermore, 2, 3, 6-trimethylphenol is important as an intermediate product for synthesis of 2, 3, 6-trimethyl-hydroquinone, a raw material for vitamin E.
Although 2, 6-xylenol is contained in coal or petroleum tar, the recovery thereof is difficult, and, accordingly, various methods of synthesis thereof have been studied.
As one of the various syntheses, there is a process for the methylation of phenol in which a phenol having at least one orthopositioned hydrogen is caused to react with methanol in the vapor phase in the presence of a catalyst which is selected from various metal oxides. For example, such a process as that wherein phenol is subjected to methylation with methanol in the vapor phase in the presence of a magnesium oxide catalyst is known.
It is generally considered that magnesium oxide catalysts are those having advantageous characteristic features, because the yield of or selectivity to methylphenols is more favorable than that in the case where the other widely known metal oxide catalysts are used. However, in the process in which magnesium oxide is employed as a catalyst, as far as we know, a high temperature and high pressure are required, and, further, the life of the catalyst is not very long.
In order to solve the difficulties as mentioned above, various improvements have been proposed, but sufficiently satisfactory results cannot be obtained by conventional improvements. For example, they are accompanied by disadvantages such as high cost of the catalyst or the continued need for high temperature and pressure even if the life of the catalyst is prolonged.